The present invention relates to the field of transmission by optical fiber, and more particularly to the field of transmission by wavelength division multiplexing (WDM) using a dispersion-shifted line fiber.
For optical fibers, the index profile is generally described as a function of the appearance of a graph showing refractive index as a function of fiber radius. Conventionally, distance r from the center of the fiber is plotted along the abscissa, and the difference between the refractive index and the refractive index of the cladding of the fiber is plotted up the ordinate. Index profiles are thus said to be xe2x80x9csteppedxe2x80x9d, xe2x80x9ctrapezoidxe2x80x9d, or xe2x80x9ctriangularxe2x80x9d for graphs that are respectively stepped, trapezoid, or triangular in shape. These curves are generally representative of the theoretical or ideal profile of the fiber, since constraints associated with manufacturing the fiber can lead to a profile that is significantly different.
In new very high data rate transmission networks that are wavelength division multiplexed, it is advantageous to generate chromatic dispersion, in particular for channel rates that are greater than or equal to 10 Gbit/s. The objective is to obtain accumulated chromatic compensation that is substantially zero over the link for all wavelength values in the multiplex, so as to limit the extent to which pulses are broadened. An accumulated value of a few hundreds of ps/nm in the dispersion is acceptable. It is also advantageous to avoid chromatic dispersion zeros in the vicinity of the wavelengths used in the system because non-linear effects are greater at such zeros. Finally, it is also advantageous to limit the chromatic dispersion slope over the range of the multiplex so as to avoid or at least limit distortion between the channels of the multiplex.
Dispersion-shifted fibers (DSF) have appeared on the market. These fibers are such that chromatic dispersion is substantially zero at the transmission wavelengths at which they are used, which wavelengths are generally not equal to the wavelength of 1.3 xcexcm at which the dispersion of silica is substantially zero. In other words the non-zero chromatic dispersion of silica is compensated by increasing the refractive index difference xcex94n between the core of the fiber and its cladding, whence the term xe2x80x9cshiftedxe2x80x9d. This difference in index makes it possible to shift the wavelength at which chromatic dispersion is zero; it is obtained by introducing dopants into the preform during manufacture thereof, for example by means of a modified chemical vapor deposition (MCVD) process of the kind that is known per se, and which is not described in greater detail below.
The term xe2x80x9cnon-zero dispersion-shifted fibersxe2x80x9d (NZ-DSF) is used to designate dispersion-shifted fibers having non-zero chromatic dispersion at the wavelengths at which they are used. For WDM optical fiber transmission systems, proposals have been made to use NZ-DSFs as line fibers, and to compensate chromatic dispersion by using sections of dispersion-compensating fiber (DCF). An example of such a transmission system is described in M. Nishimura et al., Dispersion-compensating fibers and their applications, OFC""96 Technical Digest ThA.
That solution suffers from the drawback of allowing only small power margins to be used, thereby making it difficult to operate the transmission system under realistic conditions, in particular for WDM transmission systems having a large number of channels.
Proposals have also been made to use DCF to correct the chromatic dispersion induced by a step index line fiber also known as a single-mode fiber or SMF. Such use of a dispersion-compensating fiber is mentioned in L. Grxc3xcner-Nielsen et al., Large-volume manufacturing of dispersion-compensating fibers, OFC""98 Technical Digest TuD5. A drawback of that of fiber is its high cost.
The Applicant sells one such step index monomode fiber under the reference ASMF 200. It has a chromatic dispersion canceling wavelength lying in the range 1300 nm to 1320 nm, and it also has chromatic dispersion that is not greater than 3.5 ps/(nm.km) in the range 1285 nm-1330 nm, and not greater than 17 ps/(nm.km) at 1550 nm. At 1550 nm the chromatic dispersion slope is about 0.056 ps/(nm2.km). That fiber is of the kind mentioned as a line fiber in the above-mentioned article by L. Grxc3xcner-Nielsen et al.
Compared with NZ-DSF systems having compensation using DCF, an SMF and DCF configuration has the advantage of very little penalty at high optical power, in other words it withstands non-linear effects better, and in particular it withstands four-wave mixing better. This ensures that transmission systems work properly not only under laboratory conditions, but also in the field. Nevertheless, that configuration suffers from the drawback of cost that is nearly double.
A new problem also arises for such an SMF/DCF configuration with transmission at very high data rates, typically with transmission at Nxc3x9740 Gbit/s or more, insofar as the size of the amplification-and-compensation segments does not make it possible to compensate effectively the distortion applied within each segment by dispersion and by non-linearity.
The invention proposes a solution to this new problem. More generally, the invention provides a solution to the problem of increasing channel data rate in WDM transmission systems; the solution of the invention makes it possible to retain a power margin that is compatible with operating conditions in the field. The invention also makes it possible to keep down the cost of the system.
More precisely, the invention provides a monomode optical fiber that presents, at a wavelength of 1550 nm:
an effective section area greater than or equal to 60 xcexcm2;
chromatic dispersion lying in the range 6 ps/(nm.km) to 10 ps/(nm.km), and
chromatic dispersion having a slope of absolute value less than 0.07 ps/(nm2.km).
Advantageously, the fiber has chromatic dispersion at 1550 nm in the range 7 ps/(nm.km) to 9 ps/(nm.km).
Preferably, the fiber presents chromatic dispersion greater than or equal to 7 ps/(nm.km) in the range 1530 nm to 1620 nm.
In an embodiment, the fiber has a chromatic dispersion slope at 1550 nm less than 0.05 ps/(nm2.km).
Advantageously, the fiber has an effective section area greater than or equal to 90 xcexcm2.
Preferably, the fiber has a mode radius at 1550 nm greater than 4 xcexcm.
In another embodiment, the fiber has attenuation at 1550 nm less than or equal to 0.23 dB/km.
Preferably, the fiber has polarization mode dispersion less than or equal to 0.08 ps.kmxe2x88x920.5.
In an embodiment, the fiber has an index profile in the shape of a trapezoid with a ring.
In another embodiment, the fiber has an index profile that is coaxial with a ring.
In yet another embodiment, the fiber has a coaxial index profile with a buried outer portion.
The invention also provides a WDM optical fiber transmission system having such a fiber as its line fiber.
In an embodiment, the transmission system further comprises dispersion compensation fiber.